1. Field of the Invention
The present invention relates to a clutch device with a housing containing a first fluid chamber and a second fluid chamber and with a piston of a friction clutch. The piston separates the first fluid chamber from the second fluid chamber and is axially movable with respect to a housing axis. The friction clutch can be actuated by applying a pressure difference in the fluid between the first fluid chamber and the second fluid chamber. The friction surfaces of the friction clutch are arranged in the first fluid chamber. Fluid can be supplied to the first fluid chamber and fluid can be removed from the first fluid chamber independent from the fluid pressure in the second fluid chamber in order to conduct frictional heat away from the friction clutch and its friction surfaces. Fluid passages through which fluid allocated to the first fluid chamber can flow are allocated to the friction surfaces.
2. Discussion of the Prior Art
One application of a clutch device of the type mentioned above in automobile engineering consists, for example, in that when starting by means of the clutch device a driven element, for example, the input shaft of a shift transmission, is brought as far as possible to the same speed as a drive unit, for example, an internal combustion engine. Depending on the driving torque that can be applied by the drive unit, substantial power losses occur in the clutch device, which can be designated in this connection as a xe2x80x9cstarting elementxe2x80x9d. In extreme cases, these power losses can correspond to the maximum engine output. The starting element should be able to resist such power losses even in extreme situations, for instance, when starting on a hill, specifically, with as little wear as possible over the life of the starting element.
According to a known solution, the starting element is formed by a torque converter with a frictional lockup clutch which is arranged in a drivetrain of the motor vehicle between a drive unit (internal combustion engine) and an automatic transmission and frequently has an integrated torsional vibration damper. A torque converter is extremely advantageous in many respects because it is suitable for high torques and, beyond this, can multiply torques. However, this is a comparatively complicated solution, particularly with respect to construction, since the torque converter has three parts (impeller, turbine, stator) which are rotatable relative to one another. Further, a fluid cooler (oil cooler, especially transmission cooler) associated with the torque converter must be constructed for comparatively large flows of fluid. Further, the starting behavior of the system is not alterable, which would be advantageous for optimizing the system with respect to cold starting, selecting between sports-style starting or comfortable starting, etc.
Another known solution consists in a wet clutch forming an integral component part of an automatic transmission; while this clutch can be regulated with respect to torque transmission, it is not capable of withstanding high power losses (at high engine outputs) as well, in contrast to the hydrodynamic torque converter. Further, in clutches of this kind which are integrated in the transmission relatively high friction losses occur as a result of the wet operation (Plansch operation) resulting in a correspondingly higher consumption of fuel (gasoline consumption).
Further, there also exist hydraulic clutches, or hydroclutches, as they are called, with an integrated lockup clutch which have a hydraulic circuit with an impeller wheel and a turbine wheel but without a stator wheel (and consequently without torque multiplication. Hydroclutches of this type only operate fairly economically when the hydraulic circuit is utilized only for starting and when the lockup clutch which is constructed as a friction clutch is closed as quickly as possible.
For examples of a hydroclutch and a torque converter, each with an integrated lockup clutch formed as a friction clutch, reference may be had to German Patent Application 198 28 709.7 (not yet laid open to public inspection) and DE 44 23 640 A1 which share the common feature that the clutch device is controllable via a two-line system in which an engagement state of the lockup clutch and a fluid flow through the clutch device are not adjustable independently from one another. FR 2 341 791 A1 discloses a torque converter with a lockup clutch which may be controllable via a three-line system with adjustment of an engagement state of the lockup clutch independent from adjustment of fluid flow through the converter and in which a laminated stack of the friction clutch is arranged in a fluid chamber containing a turbine wheel, a stator wheel and an impeller wheel.
A torque converter of the type mentioned above attributed to the Mercedes-Benz company is known from a German Engineers Association report entitled xe2x80x9cConstruction and Control of Slip-regulated Lockup Clutch in a Hydrodynamic Torque Converter [Aufbau und Steuerung einer schlupfgeregelten xc3x9cberbrxc3xcickungskupplung im hydrodynamischen Drehmomentwandler]xe2x80x9d, L. Hein, et. al, (VDI-Berichte No. 1175, 1995, pages 319-337). The torque converter is controllable by means of a three-line system in such a way that a fluid flow through the first fluid chamber is adjustable independently from the fluid pressure in the second fluid chamber and is accordingly adjustable independently from the engagement state or disengagement state of the friction clutch. The friction clutch of the known torque converter has a laminated stack which is arranged in the first fluid chamber and through which oil flows in an intensive manner according to information contained in the above-cited reference in order to ensure that the removal of heat from the clutch which is designed for continuous slip is good even during high transmission torques. This reference does not indicate how the claimed intensive flow of oil through the laminated stack is ensured. It appears that the flow of oil through the laminated stack depends primarily on the fluid flow forming in the first fluid chamber rather than on the supply of fluid to and removal of fluid from the first fluid chamber, since neither the report nor the drawings contained therein refers to means for positive guidance of the fluid which ensure that a determined amount of fluid having a defined relationship to the amount of fluid introduced into or removed from the first fluid chamber flows through the laminated stack when a fluid circuit is produced through the first chamber by supplying an amount of fluid to the first fluid chamber and removing a corresponding amount of fluid from the first fluid chamber.
As was found in an examination of an actual design by the Mercedes-Benz company which was familiar to experts in this technical field and which essentially corresponds to the torque converter described in the article, the Mercedes-Benz converter is provided with an oil circulation through the first fluid chamber in the following way: The oil delivered by a pump is supplied through an annular channel between a pump hub and a stator support and is fed from there between the stator wheel and the impeller shell into the first fluid chamber. Inside the converter, the supplied oil flows primarily radially outward under the influence of centrifugal force. However, because of slits in the turbine wheel, oil can also flow in the axial direction to an outlet point between the turbine hub and stator wheel, the oil being drawn off from there through an annular channel between the stator support and the driven shaft.
Oil which is pressed outward in the radial direction by centrifugal force can flow radially inward again between the house and the rear of the turbine wheel; it then strikes the laminated stack and flows partly through a gap between an outer lamination or disk carrier and the turbine wheel past the laminated stack and partly (about 10% by rough approximation) through the laminations of the laminated stack.
Before reaching the outlet point between the turbine hub and the stator wheel, the oil flowing through the laminations rejoins the oil flowing through the gap between the outer lamination carrier and the turbine wheel past the laminated stack and also rejoins any oil flowing in the axial direction through the slits in the turbine wheel before it can flow off from the first fluid chamber via the outlet point between the turbine hub and stator. In principle, it is not to be ruled out that a portion of the oil flowing through the laminated stack flows in the axial direction through the above-mentioned turbine wheel slits to the other side of the turbine wheel, where it is pushed radially outward again by the influence of centrifugal force.
Accordingly, the amount of oil flowing through the laminated stack per reference time interval depends primarily on the oil flow developing in the first fluid chamber and depends on the oil supply to the first fluid chamber and on the oil flow out of the first fluid chamber only insofar as this affects the oil flow which develops in the first fluid chamber and which depends on the operating state (rotating state) of the torque converter. Accordingly, a minimum oil quantity flowing through the laminated stack cannot be adjusted by means of a fluid circulation guided through the first fluid chamber.
Accordingly, it is the object of the present invention to provide a clutch device of the type mentioned above in which a defined minimum cooling of the friction clutch and its friction surfaces can be adjusted in view of high power losses.
This object is met, according to the invention, in that when a fluid circulation is produced through the first fluid chamber by feeding an amount of fluid into the first fluid chamber and letting a corresponding amount of fluid out of the first fluid chamber, a minimum amount of fluid flows through the fluid passages allocated to the friction surfaces, which fluid passages are in a fixed relationship with the amount of fluid introduced into and removed from the first fluid chamber. Therefore, according to the invention, it is possible to adjust a minimum fluid quantity flowing through the fluid passages and therefore a minimum cooling of the friction surfaces by means of the fluid circulation guided through the first fluid chamber, that is, the fluid quantity flowing through the first fluid chamber. This can be achieved in that at least a partial fluid circulation flows through the fluid passages, for which purpose corresponding fluid guiding means and/or distribution means can be provided which force the partial circulation flowing through the fluid passages. If there is more than one flow path for the fluid flowing through the first fluid chamber, wherein, in addition to the at least one flow path through the fluid passages, there is at least one that does not lead through the fluid passages, fluid distribution means can be provided, for example, in the form of different through-flow resistances of the fluid flow paths which adjust the fixed relationship between the total fluid quantity flowing through the first fluid chamber and the minimum fluid quantity.
According to the invention, it is possible to provide, in accordance with requirements, a cooling of the friction surfaces, possibly the laminations or disks, of the friction clutch, namely, irrespective of the actuation of the friction clutch by means of the piston. The controlling of the clutch device can be carried out for this purpose by means of a three-line system, as it is called, in which fluid can be supplied to the first fluid chamber via a first fluid line and can be removed from the first fluid chamber via a second fluid line in order to produce the fluid circulation and in which a fluid pressure which is higher or lower relative to a fluid pressure in the first fluid chamber can be applied via a third fluid line in the second fluid chamber in order to actuate the friction clutch.
According to the invention, it is possible to control the friction clutch extensively independently from the control of the first fluid chamber for forming the fluid circulation, so that it is possible to adapt the cooling volume flow to the requirements for the respective operating conditions without substantially influencing the transmission of torque of the friction clutch. This does not mean that a slight readjustment of the fluid pressure applied to the second fluid chamber would not be required in case of a change in the cooling volume flow through the first fluid chamber insofar as the change in the cooling volume flow is not required anyway by the fact that the cooling volume flow through the first fluid chamber is to be adapted to a change in the actuating state of the friction clutch, particularly to a change in the power loss occurring in the friction clutch. On the other hand, if the fluid pressure applied to the second fluid chamber is changed in order to adjust the actuation state of the friction clutch, the fluid circulation through the first fluid chamber is, as a rule, influenced insignificantly at most. Depending on the power loss which then occurs in the friction clutch, the cooling volume flow through the first fluid chamber can then be reduced or it should be increased, for example, in case of a greater slip for conducting off the increased heat loss.
Since a minimum fluid quantity which is adjustable via the fluid circulation and which cools the friction surfaces and possibly laminations is ensured according to the invention, overheated states of the friction clutch can be reliably prevented even in the event of atypical flow conditions in the first fluid chamber, so that high operating reliability and an increased service life of the friction clutch and consequently a long life of the clutch device is achieved. For this purpose, it is suggested by way of a further development that the minimum fluid quantity is 30%, preferably 50%, more preferably 70%, and most preferably 90% of the fluid quantity supplied to or removed from the first fluid chamber. The expression xe2x80x9cminimum fluid quantityxe2x80x9d employed herein allows that the fluid quantity actually flowing through the fluid passages is greater than the fluid quantity supplied to and removed from the first fluid chamber depending, for example, on flow conditions that are adjusted in the first fluid chamber and that assist in the flow of fluid through the fluid passages. It is particularly preferable that the fluid circulation passes completely or almost completely through the fluid passages, so that it is ensured that 100% or almost 100% of the fluid flowing through the first fluid chamber flows through the fluid passages.
Depending on the design of the clutch device with regard to the removal of frictional heat from the friction clutch and from the friction surfaces, that is, depending particularly on the configuration and number of fluid passages and the maximum cooling volume flow which can be guided through the latter, very high heat losses can be conducted away from the friction clutch and its friction surfaces, so that very high power losses (for example, 4 kW in continuous slip operation) and extremely high peak loss outputs (for example, up to 10 kW temporarily) can be realized.
The fluid passages can be provided between friction surfaces that can be brought into frictional engagement with one another and/or in at least one friction element having at least one of the friction surfaces. Compared with a more indirect cooling of the friction surfaces by guiding off heat via laminated carriers or the like, this construction of the clutch device ensures that the friction heat formed during the frictional engagement of the friction surfaces is guided off in a particularly effective manner.
A preferred embodiment is characterized in that the fluid passages are connected in parallel, so that fluid of essentially the same temperature is supplied to the fluid passages and substantially identical cooling conditions therefore exist for all friction surfaces. However, it is also possible to provide at least two groups of fluid passages which are connected in parallel within a group and which are connected in series from one group to another. In this case, the fluid that would flow through a group of fluid passages located downstream in the direction of flow would possibly already be heated by the frictional heat which it absorbed when flowing through preceding fluid passages beforehand. However, by providing a sufficiently large minimum fluid quantity per reference time interval, a sufficient cooling can be achieved for all friction surfaces.
With regard to the connection of fluid passage groups one after the other in series, it may be that fluid passages allocated to different friction surface pairs belong to different groups. For example, fluid passages that are allocated to friction surfaces arranged on opposite sides of a friction element can belong to one group. An example of fluid passages of this kind is channels in a lamination or disk of the friction clutch which are allocated to the friction surfaces on both sides of the lamination. Further, it is possible that fluid passages which are associated with friction surfaces that can be brought into a frictional engagement with one another belong to one group. An example for fluid passages of this kind is friction facing grooves which are associated with the friction surface having the friction facing grooves and with the friction surface which can be brought into frictional engagement with the latter and which are also open to allow fluid to flow through when friction surfaces are in frictional engagement with one another. With respect to possible embodiment forms of the channels and friction facing grooves and generally of fluid passages mentioned by way of example, reference is had to embodiment examples of the above-cited DE 44 23 640 A1 having different fluid passages of the types mentioned above.
It was already stated that at least a part of the fluid passages can be formed by friction facing grooves. Further, alternatively or in addition, at least a part of the fluid passages can be formed by grooves in at least one lamination of the friction clutch. Further, as was already stated, at least a part of the fluid passages can be formed by channels in laminations of the friction clutch which preferably have at least one smooth friction surface. As expressed herein, the channels are contrasted with the grooves in that the channels are closed in the direction transverse to the through-flow direction, that is, toward the associated friction surfaces, whereas grooves are open toward one of the associated friction surfaces in the direction transverse to the through-flow direction.
Since, as was stated, the fluid circulation through the first fluid chamber is essentially independent from the control of the friction clutch via the second fluid chamber, the geometry of the fluid passages, possibly the groove geometry and a setting (shrinkage caused by surface pressing) influencing the groove geometry or wear of the friction facings influencing the groove geometry are appreciably less critical for the operating behavior of the clutch device, especially in consideration of the cooling fluid flow through the fluid passages.
A preferred embodiment of the clutch device is characterized in that a torque transmission wheel which is mounted so as to be rotatable relative to the housing is provided in the first fluid chamber and is in a torque-transmitting connection with a torque transmission shaft and can be coupled with the housing via the friction clutch in order to produce a torque transmission connection between a drive side and a driven side of the clutch device. The first fluid chamber is preferably essentially completely filled with fluid which, in accordance with the operating state of the clutch device, is entrained by the housing and the torque transmission wheel in the rotating direction of the latter, so that substantially reduced friction losses result compared, e.g., to a Plansch operation of friction clutches integrated in a transmission. Also, the second fluid chamber is preferably essentially completely filled with fluid to prevent friction losses. The housing preferably serves as a drive side and the torque transmission shaft preferably serves as a driven side of the clutch device; the torque transmission wheel can then advisably be designated as a driven wheel and the torque transmission shaft can then be designated as a driven shaft.
The torque transmission wheel, possibly the driven wheel, can have a lamination carrying portion for at least one lamination, especially an inner lamination or outer lamination, of the friction clutch. Further, it is possible for a torsional vibration damper arrangement to be integrated in the torque transmission wheel, possibly the driven wheel, for example, with a sliding block wide-angle damper, as it is called, such as is known from DE 198 28 709.7 or in connection with dual-mass flywheel constructions by the present Applicant.
There are different possibilities for providing the minimum fluid quantity flowing through the fluid passages when the fluid circulation is produced. According to one solution, the torque transmission wheel divides the first fluid chamber into two fluid chamber areas which communicate via the fluid passages so as to allow fluid flow, and, when the fluid circulation is produced, one of the fluid chamber areas serves as a fluid supply area supplying fluid to the fluid passages and the other serves as a fluid outlet area which guides fluid out of the fluid passages. The two fluid chamber areas preferably communicate in a flow connection principally or essentially solely through the fluid passages (aside from some kind of fluid flow connection via an associated fluid supply such as a fluid pump with a fluid reservoir) and can be connected in a manner known per se via axially extending fluid channels (for example, annular channels between shafts of the clutch device) to a fluid supply.
If the torque transmission wheel has a torsional vibration damper arrangement, appropriate seals should be provided in the area of the torsional vibration damper arrangement to prevent flows of fluid between the two fluid chamber areas which would bridge the fluid passages. In this connection, it is highly advisable when a damper cage of the torsional vibration damper having at least one torsional spring element is constructed as a substantially closed cage.
According to a further solution for providing the fluid flow according to the invention through the fluid passages, an intermediate wall is provided in the first fluid chamber, possibly adjacent to the torque transmission wheel, which intermediate wall (possibly in cooperation with one or more other components of the clutch device) divides the first fluid chamber into two fluid chamber areas which communicate via the fluid passages so as to permit a flow of fluid, and, when the fluid circulation is produced, one of the fluid chamber areas serves as a fluid supply area supplying fluid to the fluid passages and the other serves as a fluid outlet area which guides fluid out of the fluid passages. The intermediate wall can be connected with the housing so as to be fixed with respect to rotation relative to it and/or can be axially secured to the housing and preferably has at least one friction surface of the friction clutch. The two fluid chamber areas (aside from any fluid flow connection via the associated fluid supply) are preferably in a fluid flow connection chiefly or substantially exclusively via the fluid passages.
According to another solution for achieving the flow through the fluid passages according to the invention, the housing has a fluid channel arrangement by which, in order to produce the fluid circulation, fluid can be supplied to an area of the first fluid chamber located radially outside of the friction surfaces or by which fluid can be conducted away from this area in order to provide a fluid flow of the fluid circulation passing through an area of the first fluid chamber located radially inside of the friction surfaces, through the fluid passages, through the area of the first fluid chamber located radially outside of the friction surfaces, and through the fluid channel arrangement.
In order to achieve the largest possible fluid flow through the fluid passages, a connection area between a lamination of the friction clutch and an associated lamination carrier, especially a toothing of the lamination with the lamination carrier, can be protected against a substantial flow of fluid by-passing the fluid passages. For this purpose, it is suggested as a particularly simple and therefore economical (and nevertheless reliable) step for securing that at least one sealing ring is arranged between two laminations or between a lamination and a surface of the housing or piston located opposite to the lamination.
In connection with the construction of the torque transmission wheel with a lamination carrying portion, it is suggested in a further development that a lamination is arranged at the lamination carrying portion so as to be fixed with respect to axial displacement, preferably in such a way that the connection area between the lamination and the lamination carrying portion is protected against a substantial flow of fluid by-passing the fluid passages. In this connection, it is also extremely advantageous when a hub of the torque transmission wheel in the housing has play for axial displacement on the torque transmission shaft. Axial bearings located on the radial inside can be omitted in this case because an axial positioning is achieved by means of the lamination which is fixed with respect to axial displacement at the lamination carrying portion. All of this results in advantages with respect to cost and savings in axial installation space which can be made available for the fluid circulation for radial through-flow with a large effective flow cross section.
The clutch device according to the invention can be constructed in such a way that, apart from the friction clutch, it has essentially no additional torque transmission connection between a drive side (possibly the housing) and a driven side (possibly the driven shaft with the driven wheel) of the clutch device. In this case, the torque transmitted by the clutch device can be adjustable via an appropriate actuation of the friction clutch, possibly by adjusting a defined slip. In any case, a controllable or regulatable slip can be dispensed with in a torsional vibration damper arrangement.
However, the clutch device can have, in addition to the friction clutch, a hydrodynamic circuit as a further torque transmission connection between a drive side (possibly housing) and a driven side (possibly driven shaft with driven wheel) of the clutch device. In this case, the torque transmission wheel (driven wheel) can be allocated as a turbine wheel, or a separate turbine wheel, which forms the hydrodynamic circuit together with an impeller wheel possibly formed by a housing portion and, as the case may be, with a stator wheel. Accordingly, the clutch device can also be a torque converter (construction of the hydrodynamic circuit with stator wheel) or a hydroclutch (construction of hydrodynamic circuit without stator wheel) which does not enable torque amplification, wherein the friction clutch serves as a lockup clutch in both cases. Also, in the case of the torque converter or hydroclutch, the slip of the lockup clutch can be controllable, for example, in order to dispense with a separate torsional vibration damper or a torsional vibration damper integrated in the clutch device.
A construction with the intermediate wall, already mentioned, is especially advisable for the clutch device with hydrodynamic circuit. The intermediate wall can be arranged axially between the torque transmission wheel (particularly the driven wheel) and the piston. It is especially preferable that the turbine wheel itself serves as driven wheel.
In a further development, it is suggested that the intermediate wall is connected in a radial inner area with a radial inner housing portion, possibly with a supporting ring or housing hub, for common rotation with the housing so as to be fixed with respect to rotation relative to it and is preferably fixedly connected axially, wherein a fluid channel arrangement is provided in the housing portion and/or between the housing portion and the intermediate wall and/or in a hub portion of the intermediate wall, wherein, via this fluid channel arrangement, a fluid chamber area of the two fluid chamber areas which is defined by the piston and the intermediate wall and which preferably serves as a fluid inlet area is connected to or can be connected to an axially extending fluid channel arrangement which is connected to or can be connected to a fluid source or a fluid reservoir.
The intermediate wall can have a radially outer circumference at a radial distance from a radially outer circumferential wall portion of the housing. In this respect, at least one driven-side lamination arranged axially between the piston and the intermediate wall can be held in a radial area of the first fluid chamber between the outer circumference and the circumferential wall portion in a torque transmission connection with the driven side of the clutch device, possibly by means of at least one lamination carrying portion of the torque transmission wheel, possibly the turbine wheel.
The piston can extend radially up to the circumferential wall portion of the housing and can be guided at that location in a sealed manner. In this way, a particularly large effective surface of the piston is offered to the fluid in the second fluid chamber and consequently a particularly high transmission capability of the friction clutch can be achieved.
It has already been mentioned that the fluid chamber area which is defined by the intermediate wall and the piston (possibly in cooperation with at least one further component of the clutch device) can serve as a fluid inlet area. This has the effect that the oil supplied to the first fluid chamber flows through the fluid passages in the cool state (possibly proceeding from a transmission cooling unit) and only then does it flow through the hydrodynamic circuit, which likewise heats the oil, into the other fluid chamber area serving as fluid outlet area, so that the cooling of the friction clutch and its friction surfaces is particularly effective. The disadvantage caused by this flow direction, namely, that a suction action of the impeller wheel cannot be utilized and must be pumped to a certain extent against the suction action of the impeller wheel, can certainly be tolerated to gain the advantage mentioned above. If desired, an opposite fluid flow direction can also be provided through the first fluid chamber, for example, in order to utilize the suction action of the impeller wheel for producing the fluid circuit through the first fluid chamber.
It should already have been made clear that, according to the invention, the piston of the friction clutch is controllable independently from the fluid flow through the first fluid chamber, wherein the friction clutch can have the following operating states: open (disengaged), slipping and completely engaged (fully engaged lockup). For this purpose, the clutch device can be controllable via a three-line system, as it is called. Due to the fact that the piston is controlled independently from the fluid circuit through the first fluid chamber, it is possible to adjust the fluid flow through the first fluid chamber in the open and slipping state of the friction clutch (lockup clutch, possibly converter clutch) corresponding to the power loss resulting from the engaged state of the clutch (engine torque modified by slip speed). When the friction clutch is completely engaged (zero slip speed), the fluid circuit through the first fluid chamber can and should be interrupted in order to minimize the losses through an associated fluid pump, possibly transmission pump.
A slip operation of the friction clutch, possibly a lockup clutch or converter clutch, is useful in different respects. Accordingly, a slipping friction clutch in which a differential speed occurs between a drive side and a driven side of the clutch device can be used to decouple the driven side from the drive side for torsional vibrations of a drive unit, that is, to damp torsional vibrations, wherein fluctuations in both torque and moment are suppressed or damped. Accordingly, a torsional vibration damper arrangement with torsional damper springs can be dispensed with in some cases. This applies to a clutch device with a hydrodynamic circuit as well as to a clutch device without hydrodynamic circuit.
In the case of a clutch device with a hydrodynamic circuit, especially in the case of a torque converter, a torque which is supportable by the clutch device can be increased relative to the torque that is supportable by a pump or an impeller alone by means of the slipping lockup clutch. For example, a converter having a so-called xe2x80x9csoft characteristicxe2x80x9d can be made xe2x80x9chardxe2x80x9d by the slipping converter clutch in that the lockup clutch supports an additional torque of the engine or the like aside form the torque supported by the impeller. Depending on the torque that can be exerted by an engine, a so-called soft characteristic could be present, for example, when the impeller wheel can support approximately 80 Nm at a speed of 2000 RPM. When the engine can exert an appreciably greater torque, xe2x80x9chowlingxe2x80x9d occurs in the engine, e.g., when starting, until a speed is reached at which the torque that can be applied by the engine can be supported by the torque converter. A howling of the engine enables a fast rise of the torque transmitted to the transmission, but results in increased gasoline consumption. In the case of a converter with a so-called xe2x80x9chard characteristicxe2x80x9d in which the pump can support 200 Nm, for example, at a speed of 2000 RPM, the speed for supporting the engine torque would rise only to a noticeably small degree if at all, resulting in a more economical and comfort-oriented driving style. As a result of the slipping lockup clutch, a converter which is xe2x80x9csoftxe2x80x9d, per se, due to the design of the hydrodynamic circuit can be made xe2x80x9chardxe2x80x9d, wherein the torque which can be supported for a reference speed of the converter can be adjusted by adjusting the engagement state of the friction clutch, that is, by adjusting the torque which is transmitted by the friction clutch and which bypasses the hydrodynamic circuit. Therefore, both sporty driving and economical comfort-oriented driving are made possible, as desired, by means of the same drivetrain.
As was already mentioned, because of the good cooling capability provided according to the invention for the friction surfaces with adjustment of a minimum fluid quantity, especially high power losses of the friction clutch can be realized in the case of a torque converter of the converter clutch or lockup clutch. In this connection, the above-mentioned power losses of 4 kW in continuous slip operation and 10 kW in case of peak loads may be sufficient in practice, in any case when assuming a typical power loss of the hydrodynamic circuit in the non-bypassed state in the amount of about 10 kW to 15 kW. The magnitude of power loss for which the friction clutch (lockup clutch) is to be designed is determined by the engine torque to be transmitted and the maximum slip speed in continuous slip operation, wherein an increased power loss can be caused by an engine with a higher torque and/or by an increased slip speed (for greater damping of torsional vibrations and/or for modification of the characteristic of the converter). For example, at an engine torque of 200 Nm and a slip speed of 100 RPM, there is a power loss of 2 kW which is to be guided off from the friction clutch and its friction surfaces primarily through the fluid circuit through the first fluid chamber. The doubling of the slip speed would then lead to a power loss of 4 kW. The same applies to an increase in the engine torque. Because of the good cooling of the friction clutch and its friction surfaces which can be realized according to the invention and because of the resulting good ability to manage high power losses, it is possible to design the clutch device and its friction clutch for a wide variety of different requirements.
The clutch device can be designed as a motor vehicle starting element which serves to match a drive speed with a driven speed when the motor vehicle is started. Particularly when the clutch device according to the invention is to be used for this purpose, it is advisable to provide no torque transmission connection between the drive side and the driven side of the clutch device other than the friction clutch.
In general, it is suggested that the clutch device according to the invention is constructed as a separate constructional unit which can be installed in a drivetrain of a motor vehicle between a drive unit and a transmission.
The invention is further directed especially to a clutch device which can be installed in a drivetrain of a motor vehicle between a drive unit and a transmission, with a housing containing a first fluid chamber and a second fluid chamber, and with a piston of a friction clutch, which piston separates the first and second fluid chamber from one another and is axially movable with respect to a housing axis, wherein the friction clutch can be actuated by the application of a fluid pressure difference between the first fluid chamber and the second fluid chamber, wherein the friction surfaces of the friction clutch are arranged in the first fluid chamber, and serves to produce a torque transmission connection between an input side and an output side of the clutch device, wherein fluid can be supplied to the first fluid chamber and fluid can be removed from the first fluid chamber independently from the fluid pressure in the second fluid chamber in order to conduct frictional heat away from the friction clutch and its friction surfaces. It is suggested, according to the invention, for this clutch device, which is preferably constructed as a separate constructional unit, that the clutch device has essentially no other torque transmission connection between the drive side and the driven side of the clutch device apart from the friction clutch.
As was already mentioned above with respect to the clutch device according to the first aspect of the invention, a clutch device of this kind can be used particularly advantageously as a motor vehicle starting element which serves to adjust or match a drive speed and a driven speed when starting the motor vehicle. The clutch device according to the invention makes it possible to adjust the starting behavior of the system in a specific manner by applying a corresponding fluid differential pressure between the first and second fluid chambers. The starting behavior of the system can therefore be adjusted, for example, for cold starting as well as for sporty or comfortable starting, etc.
In other respects, the clutch device according to the second aspect of the invention can be constructed like the above-mentioned clutch device according to the first aspect of the invention; with respect to a high durability of the clutch device, a minimum fluid quantity above all should be adjustable by means of a fluid circuit passing through the first fluid chamber, this minimum fluid quantity flowing through the fluid passages associated with the friction surfaces.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.